Thin film transistor substrate, method for manufacturing the same, and liquid crystal display

ABSTRACT

A thin film transistor substrate includes a semiconductor channel layer made of an oxide semiconductor, protective insulating layers that cover the semiconductor channel layer, a first source electrode, a first drain electrode, a second source electrode, and a second drain electrode. The second source electrode is located on the first source electrode and connected with the semiconductor channel layer through a first contact hole. The second drain electrode is located on the first drain electrode and connected with the semiconductor channel layer through a second contact hole.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a thin film transistor substrate, amethod for manufacturing the same, and a liquid crystal display.

Description of the Background Art

Thin film transistor active matrix substrates (thin film transistorsubstrates (TFT substrates)) including thin film transistors (TFTs) asswitching elements find use in electro-optical apparatuses such asdisplays (liquid crystal displays) including liquid crystals. Thesemiconductor devices such as the TFTs have features of beingpower-thrifty and thin, thus being used in place of cathode ray tubes(CRTs) and finding increasing applications to flat panel displays.

The electro-optical elements for use in liquid crystal displays(hereinafter also referred to as “LCDs”) include passive matrix LCDs andTFT LCDs including TFTs as switching elements. In particular, the TFTLCDs are superior in portability and display quality to the CRTs and thepassive matrix LCDs, thus finding widespread practical applications todisplay products such as notebook computers and TVs.

In general, the TFT LCD includes a liquid crystal display panel in whicha liquid crystal layer is sandwiched between a TFT substrate and acounter substrate. The TFT substrate includes an array of a plurality ofTFTs and the counter substrate includes, for example, a color filter.The liquid crystal display panel includes polarizing plates located onthe front surface side and the back surface side and a backlight locatedon one of these sides. This structure provides an excellent colordisplay.

The LCDs employing the in-plane switching (IPS) (registered trademark)mode have the advantage in wide viewing angle and thus are widely usedin, for example, displays. The IPS mode is the transverse electric fieldliquid crystal driving mode yielding an improvement in the viewing angleof the conventional TFT LCDs. Unfortunately, such LCDs have a lowaperture ratio and a low transmittance in the pixel display portion, andthus, can hardly have bright display properties. It is considered thatthe following factor plays a major role in interfering with the brightdisplay properties. The electric field for driving liquid crystals failsto work properly above the comb-teeth shaped pixel electrode included inthe IPS LCD, so that a part of the liquid crystals located above thepixel electrode is not driven. To solve this problem, the LCD disclosedin, for example, Japanese Patent Application Laid-Open No. 2001-56474has been proposed which employs the fringe field switching (FFS) mode.

For the switching element of the TFT substrate included in theconventional liquid crystal display, amorphous silicon (Si) has beenused as the semiconductor film being the active layer (channel layer).In recent years, the TFTs including an active layer made of an oxidesemiconductor have been actively developed. Such oxide semiconductor hasa higher mobility than that of the conventionally-used amorphoussilicon. The oxide semiconductor is mainly the material based on zincoxide (ZnO) or the material based on amorphous InGaZnO obtained byadding gallium oxide (Ga₂O₃) and indium oxide (In₂O₃) to zinc oxide.This technique is disclosed in, for example, Japanese Patent ApplicationLaid-Open No. 2001-56474, Japanese Patent Application Laid-Open No.2005-77822, and Kenji Nomura, et al., “Room-temperature fabrication oftransparent flexible thin-film transistors using amorphous oxidesemiconductors,” Nature, no. 432 (2004): 488-492.

Similarly to the oxide conductors being the transparent conductors suchas amorphous ITO (indium oxide (In₂O₃)+tin oxide (SnO₂)) and amorphousInZnO (indium oxide (In₂O₃)+zinc oxide (ZnO)), the above-mentioned oxidesemiconductor can be etched with a weak acid solution containing oxalicacid or carboxylic acid, thus having the advantage of being easilypatterned.

However, in some cases, the oxide semiconductor is damaged by the acidsolution used to etch a general metal film which is to be formed into asource electrode and a drain electrode of the TFT, resulting incharacteristics degradation. The oxide semiconductors made of certainkinds of material may dissolve in the above-mentioned acid solution.Thus, as described in Japanese Patent Application Laid-Open No.2007-281409, for the manufacturing of the TFT (generally referred to asback channel etch (BCE) TFT) including the source electrode and thedrain electrode as an upper layer on the oxide semiconductor film, theoxide semiconductor film being the channel layer is damaged by the acidsolution in some cases during the processing of the source electrode andthe drain electrode, resulting in the degradation of the TFTcharacteristics. In other cases, the oxide semiconductor film being thechannel layer is damaged due to the oxidation-reduction reaction in theinterface during the deposition of the metal film which is to be formedinto the source electrode and the drain electrode, causing thedegradation of the TFT characteristics.

These problems can be solved by applying the TFT structure thatincludes, as an upper layer on the semiconductor layer, a protectivelayer to protect the semiconductor layer as described in Japanese PatentApplication Laid-Open No. 62-235784 (1987) and International PublicationNo. WO 2011/077607. Such TFT structure can prevent or reduce damage tothe oxide semiconductor film during the etching of the metal film whichis to be formed into the source electrode and the drain electrode orloss of the oxide semicondcutor resulting from the etching. Thisstructure is generally referred to as “etching stopper (ES) TFT” or“etch stopper (ES) TFT” as a reflection of the above-mentioned function.

The application of the etch stopper TFT structure to the TFTs of thegeneral FFS LCD disclosed in, for example, Japanese Patent ApplicationLaid-Open No. 2001-56474 requires an additional photolithography processdedicated to the formation of the protective layer on the oxidesemiconductor layer. Unfortunately, the additional photolithographyprocess required in the manufacturing process reduces the productioncapacity and increases the manufacturing cost. The increasedmanufacturing cost resulting from the application of the etch stopperTFT structure is the problem associated not only with the FFS LCDs butalso with other LCDs in which the TFTs including the active layer beingthe oxide semiconductor serve as switching elements.

If the general metal film which is to be formed into the sourceelectrode and the drain electrode is deposited directly onto the oxidesemiconductor film by sputtering or chemical vapor deposition in thestate where the channel layer (region) of the semiconductor film isprotected by the etch stopper layer, the interface reaction (theoxidation-reduction reaction) would occur in the source region and thedrain region of the semiconductor film that are electrically connectedwith the source electrode and the drain electrode. If this is the case,the oxidation-reduction reaction in the oxide semiconductor film wouldhave a spread effect across the semiconductor layer including thechannel region, resulting in the degradation of the TFT characteristics.

SUMMARY OF THE INVENTION

The present invention therefore has been made in view of theabove-mentioned problems, and an object thereof is to provide atechnique capable of improving the characteristics of a thin filmtransistor.

The present invention is a thin film transistor substrate including aninsulating substrate, a gate electrode, a first insulating film, asemiconductor film formed of an oxide semiconductor, a first sourceelectrode formed of a conductor other than oxide conductors, a firstdrain electrode formed of a conductor other than oxide conductors, asecond source electrode formed of an oxide conductor, a second drainelectrode formed of an oxide conductor, a pixel electrode, and a thirdinsulating film. The gate electrode is located on the insulatingsubstrate. The first insulating film covers the gate electrode. Thesemiconductor film is located over the gate electrode with the firstinsulating film located therebetween. The second insulating film coversthe semiconductor film. The first source electrode is located on one endof the second insulating film. The first drain electrode is located onanother end of the second insulating film. The second source electrodeis located on the first source electrode and connected with thesemiconductor film through a first contact hole provided in the firstsource electrode and the second insulating film. The second drainelectrode is located on the first drain electrode and connected with thesemiconductor film through a second contact hole provided in the firstdrain electrode and the second insulating film. The pixel electrode isformed by extending a part of the second drain electrode. The thirdinsulating film covers the first insulating film, the second sourceelectrode, the second drain electrode, and the pixel electrode.

The present invention is capable of improving the characteristics of thethin film transistor.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the configuration of a TFT substrate accordingto a preferred embodiment;

FIG. 2 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment;

FIG. 3 is a plan view of the configuration of the TFT substrateaccording to the preferred embodiment after a first photolithographyprocess;

FIG. 4 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment after the firstphotolithography process;

FIG. 5 is a plan view of the configuration of the TFT substrateaccording to the preferred embodiment after a second photolithographyprocess;

FIG. 6 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment after the secondphotolithography process;

FIG. 7 is a plan view of the configuration of the TFT substrateaccording to the preferred embodiment after a third photolithographyprocess;

FIG. 8 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment after the thirdphotolithography process;

FIG. 9 is a plan view of the configuration of the TFT substrateaccording to the preferred embodiment after a fourth photolithographyprocess;

FIG. 10 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment after the fourthphotolithography process;

FIG. 11 is a plan view of the configuration of the TFT substrateaccording to the preferred embodiment after a fifth photolithographyprocess;

FIG. 12 is a cross-sectional view of the configuration of the TFTsubstrate according to the preferred embodiment after the fifthphotolithography process;

FIG. 13 is a plan view of the configuration of a TFT substrate accordingto a modification; and

FIG. 14 is a cross-sectional view of the configuration of the TFTsubstrate according to the modification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred Embodiment

With reference to FIGS. 1 and 2, the following describes theconfiguration of a TFT substrate according to this preferred embodiment.FIG. 1 is a view of the planar configuration of a pixel portion as wellas a gate terminal portion and a source terminal portion outside thepixel portion (pixel display region) of the TFT substrate in the FFSmode. FIG. 2 is a view of the cross-sectional configuration of theseportions. FIG. 2 shows cross sections taken along the line X1-X2, theline Y1-Y2, and the line Z1-Z2 in FIG. 1. The cross section taken alongthe line X1-X2 corresponds to the region (the pixel portion) in whichpixels are located. The cross section taken along the line Y1-Y2corresponds to the region (the gate terminal portion) in which a gateterminal 4 and a gate terminal pad 25 for supplying a gate signal to agate wire 3 (not shown in FIG. 2) are located. The cross section takealong the line Z1-Z2 corresponds to the region (the source terminalportion) in which source terminals 19 a and 19 b and a source terminalpad 26 for applying a display signal to source wires 18 a and 18 b (notshown in FIG. 2) are located.

As shown in FIG. 2, the pixel portion includes, along the X1-X2 line, aTFT portion being the region in which the TFT is located, a transmissivepixel portion being the region in which a pixel electrode 20 and acounter electrode 27 are located, and a common electrode connectionportion in which the counter electrode 27 and a common wire 5 areconnected.

The TFT substrate according to this preferred embodiment includes aninsulating substrate 1, a gate electrode 2, the common wire 5, a firstinsulating film 6, a semiconductor channel layer 7 being a semiconductorfilm, protective insulating layers 9, 10, and 11 being a secondinsulating film, a first source electrode 12, a first drain electrode13, a second source electrode 16, a second drain electrode 17, the pixelelectrode 20, a third insulating film 21, and the counter electrode 27.The following describes these constituent components and the remainingconstituent components (such as the gate wire 3) of the TFT substratesaccording to this preferred embodiment.

The insulating substrate 1 is the transparent insulating substrate madeof, for example, glass. The gate electrode 2, the gate wire 3, the gateterminal 4, and the common wire 5 are located on the insulatingsubstrate 1. The first insulating film 6 covers the gate electrode 2,the gate wire 3, the gate terminal 4, and the common wire 5. The firstinsulating film 6 functions as the gate insulating film in the TFTportion, and thus, is hereinafter also referred to as “gate insulatingfilm 6.”

As shown in FIG. 1, the gate wire 3 extends in the lateral direction andis electrically connected with the gate electrode 2 and the gateterminal 4. The segment of the gate wire 3 located in the TFT portion (apart of the gate wire 3) is the gate electrode 2. The gate electrode 2has a width greater than that of the remaining part of the gate wire 3.The segment of the gate wire 3 located in the gate terminal portion (theend portion of the gate wire 3) is the gate terminal 4. The common wire5 extends in parallel with the gate wire 3. The first insulating film 6and the third insulating film 21 may be omitted as appropriate in FIG. 1and the subsequent plan views.

As shown in FIGS. 1 and 2, the TFT portion of the TFT substrateaccording to this preferred embodiment includes the gate electrode 2,the gate insulating film 6 (the first insulating film 6), thesemiconductor channel layer 7, the protective insulating layer 9, thefirst source electrode 12, the first drain electrode 13, the secondsource electrode 16, and the second drain electrode 17. The electrode 2and the gate insulating film 6 are as described above.

The semiconductor channel layer 7 being the semiconductor film islocated over the gate electrode 2 with the gate insulating film 6located therebetween. In this preferred embodiment, the semiconductorchannel layer 7 is located inside the periphery of the gate electrode 2in plan view.

The semiconductor channel layer 7 is made of an oxide semiconductor. Theoxide semiconductor provided as the semiconductor channel layer 7 maybe, for example, the oxide semiconductor based on zinc oxide (ZnO), theoxide semiconductor based on InZnSnO obtained by adding indium oxide(In₂O₃) and tin oxide (SnO₂) to zinc oxide, or the oxide semiconductorbased on InGaZnO obtained by adding gallium oxide (Ga₂O₃) and indiumoxide (In₂O₃) to zinc oxide. The semiconductor channel layer 7 made ofthe oxide semiconductor offers an improved mobility over thesemiconductor channel layer 7 made of amorphous silicon.

The protective insulating layers 9, 10, and 11 being the secondinsulating film cover the semiconductor channel layer 7. The protectiveinsulating layer 10 is located on the upper part and the side part ofone end of the semiconductor channel layer 7. The protective insulatinglayer 11 is located on the upper part and the side part of another endof the semiconductor channel layer 7. The protective insulating layer 9is located on the semiconductor channel layer 7 between the protectiveinsulating layer 10 and the protective insulating layer 11. Thefollowing description is given assuming that the protective insulatinglayer 9 is continuous with the protective insulating layers 10 and 11.Alternatively, the protective insulating layer 9 may be separated fromthe protective insulating layers 10 and 11.

The first source electrode 12 is located on the protective insulatinglayer 10 and the end portion of the protective insulating layer 9 on theprotective insulating layer 10 side, or equivalently, on one end of thesecond insulating film. The first drain electrode 13 is located on theprotective insulating layer 11 and another end portion of the protectiveinsulating layer 9 on the protecting insulating layer 11 side, orequivalently, on another end of the second insulating film.

The first source electrode 12 and the first drain electrode 13 are madeof a conductor other than oxide conductors and are separated from eachother. In this preferred embodiment, the first source electrode 12 andthe first drain electrode 13 are made of a conductor being a metal or analloy (for example, the alloy obtained by adding trace amounts ofanother element to each of molybdenum (Mo) and aluminum (Al)).

The second source electrode 16 is located on the first source electrode12 and connected with the semiconductor channel layer 7 through a firstcontact hole 14 provided in the first source electrode 12 and theprotective insulating layers 9, 10, and 11. The second source electrode16 is made of an oxide conductor (such as an ITO film containing In₂O₃and SnO₂). In this preferred embodiment, the first contact hole 14 isprovided between the protective insulating layer 9 and the protectiveinsulating layer 10.

Similarly, the second drain electrode 17 is located on the first drainelectrode 13 and connected with the semiconductor channel layer 7through a second contact hole 15 provided in the first drain electrode13 and the protective insulating layers 9, 10, and 11. The second drainelectrode 17 is made of an oxide conductor (such as an ITO filmcontaining In₂O₃ and SnO₂). In this preferred embodiment, the secondcontact hole 15 is provided between the protective insulating layer 9and the protective insulating layer 11.

The following describes the constituent components of the TFT substrateother than the TFT portion.

As shown in FIG. 1, the first source electrode 12 is located so as tobranch off from the source wire 18 a. Similarly, the second sourceelectrode 16 on the first source electrode 12 is located so as to branchoff from the source wire 18 b on the source wire 18 a and the protectiveinsulating layer 10 below the first source electrode 12 is located so asto branch off from a protective insulating layer 8 below the source wire18 a. The end portions of the source wires 18 a and 18 b are providedwith the source terminals 19 a and 19 b, respectively.

As shown in FIG. 2, the pixel electrode 20 is connected with the seconddrain electrode 17 and located on the first insulating film 6. In thispreferred embodiment, the pixel electrode 20 is formed by laterallyextending a part of the second drain electrode 17.

The third insulating film 21 covers the first insulating film 6 on thecommon wire 5, the second source electrode 16, the second drainelectrode 17, and the pixel electrode 20. In this preferred embodiment,the third insulating film 21 also covers, for example, the firstinsulating film 6 on the gate terminal 4 and the protective insulatinglayer 9.

The gate terminal pad 25 is electrically connected with the gateterminal 4 through a gate contact hole 22 provided in the firstinsulating film 6 and the third insulating film 21. The source terminalpad 26 is electrically connected with the source terminals 19 a and 19 bthrough a source contact hole 23 provided in the third insulating film21.

The counter electrode 27 is located over the pixel electrode 20 with thethird insulating film 21 located therebetween. The counter electrode 27is connected with the common electrode 5 through a common electrodecontact hole 24 (a third contact hole) provided in the first insulatingfilm 6 and the third insulating film 21. The counter electrode 27 has aslit opening 27 a. Thus, when a voltage is applied between the pixelelectrode 20 and the counter electrode 27, the counter electrode 27 cangenerate a transverse electric field between the pixel electrode 20 andthe counter electrode 27 above the counter electrode 27.

<Manufacturing Method>

With reference to FIGS. 3 to 14, the following describes a method formanufacturing the TFT substrate according to this preferred embodiment.In FIGS. 3 to 14, the constituent components corresponding to theconstituent components shown in FIGS. 1 and 2 are denoted by the samereference signs in FIGS. 1 and 2.

Firstly, the insulating substrate 1 is cleaned with cleaning fluid orpure water. In this preferred embodiment, the glass substrate having athickness of 0.5 mm was used as the insulating substrate 1. Then, afirst conductive film being the material of, for example, the gateelectrode 2 and the gate wire 3 is deposited on the entirety of one mainsurface of the cleaned insulating substrate 1.

The first conductive film may be, for example, a general metal, such aschromium (Cr), molybdenum (Mo), titanium (Ti), copper (Cu), tantalum(Ta), tungsten (W) or aluminum (Al), or an alloy obtained by adding atleast one different element to the above-mentioned metallic elementbeing the base component. The base component element is the element ofthe highest content among the elements contained in the alloy. The firstconductive film may have a laminated structure including at least twolayers of such metal or alloy. A low-resistant conductive film having aspecific resistance value of 50 μΩcm or less can be made of such metalor alloy. In this preferred embodiment, an aluminum (Al) alloy filmhaving a thickness of 200 nm was deposited as the first conductive filmby sputtering, using argon (Ar) gas.

Then, a resist material is applied onto the Al alloy film. In a firstphotolithography process, the resist material is formed into aphotoresist pattern and the Al alloy film is selectively etched to bepatterned, using the photoresists pattern as a mask. The Al alloy filmmay be wet-etched with a solution (PAN chemical solution) containing,for example, phosphoric acid, acetic acid, and nitric acid. Then, thephotoresist pattern is removed.

Consequently, as shown in FIGS. 3 and 4, the gate electrode 2, the gatewire 3, the gate terminal 4, and the common wire 5 on the insulatingsubstrate 1 are formed by patterning the first conductive film.

Next, a first to-be-processed insulating film being the material of thefirst insulating film 6 is deposited on the entire upper surface of theinsulating substrate 1. In this preferred embodiment, a silicon oxide(SiO) film and a silicon nitride (SiN) film were formed as the firstto-be-processed insulating film by chemical vapor deposition (CVD). Thesilicon oxide film having a thickness of 50 nm and the silicon nitridefilm having a thickness of 400 nm were deposited. The silicon oxide filmhas poor barrier properties (shielding properties) against impurityelements affecting the TFT characteristics, such as water (H₂O),hydrogen (H₂), sodium (Na), and potassium (K). Thus, the firstto-be-processed film was formed into a laminated structure including,for example, SiN having excellent barrier properties as the lower layerbelow SiO. The majority of the first to-be-processed film is to beformed into the first insulating film 6 and the gate insulating film 6,and therefore, the first to-be-processed film is referred to as the gateinsulating film 6 or the first insulating film 6 in the followingdescription.

Then, an oxide semiconductor film being the material of thesemiconductor channel layer 7 is deposited on the first insulating film6. In this preferred embodiment, the oxide semiconductor of the film isthe oxide (such as InGaZnO) containing In, Ga, and Zn. As an example, anInGaZnO oxide semiconductor film was deposited by sputtering, using anIn—Ga—Zn—O target [In₂O₃.Ga₂O₃.(ZnO)₂] in which the atomic compositionratio of In:Ga:Zn:O is 1:1:1:4. The oxide semiconductor film depositedas described above usually has an atomic composition ratio of oxygenthat is smaller than the stoichiometry. Consequently, the oxidesemiconductor film becomes deficient in oxygen ion (the compositionratio of O is less than 4 in the above example). Thus, Ar gas mixed withoxygen (O₂) gas is preferably used for sputtering. In this preferredembodiment, a sputtering was performed using Ar gas containing O₂ gasmixed at 10% by partial pressure ratio, whereby an InGaZnO film having athickness of 50 nm was deposited. The InGaZnO film formed as describedabove has an amorphous structure. The InGaZnO film having the amorphousstructure usually has a crystallization temperature of 500° C. orhigher. Thus, at room temperature, the majority of the InGaZnO film hasthe amorphous structure with stability.

Next, in a second photolithography process, a photoresist pattern isformed and the oxide semiconductor film is selectively etched to bepatterned, using the photoresist pattern as a mask. The oxidesemiconductor film may be wet-etched with a chemical solutioncontaining, for example, carboxylic acid. The chemical solutioncontaining carboxylic acid preferably has an oxalic acid content of, forexample, 1 to 10 wt %. In this preferred embodiment, the oxidesemiconductor film was etched with an oxalic-acid-based chemicalsolution containing 5 wt % oxalic acid and water. Then, the photoresistpattern is removed.

Consequently, as shown in FIGS. 5 and 6, the semiconductor channel layer7 located over the gate electrode 2 with the first insulating film 6located therebetween is formed by patterning the oxide semiconductorfilm. The portion of the first insulating film 6 sandwiched between thegate electrode 2 and the semiconductor channel layer 7 functions as thegate insulating film 6.

Next, a second to-be-processed insulating film being the material of theprotective insulating layers 8, 9, 10, and 11 is deposited on the firstinsulating film 6 and the semiconductor channel layer 7. In thispreferred embodiment, a SiO film having a thickness of, for example, 100nm was formed as the second to-be-processed film by CVD.

Subsequently, a second conductive film is deposited on the secondto-be-processed insulating film. In this preferred embodiment, as thesecond conductive film, a laminated structure was formed by laminatingthe alloy obtained by adding trace amounts of another element to each ofmolybdenum (Mo) and aluminum (Al).

Next, in a third photolithography process, a photoresist pattern isformed, and the second conductive film (the film including a laminationof Al alloy and Mo alloy) is selectively etched to be patterned, usingthe photoresist pattern as a mask. The second conductive film may bewet-etched with a solution (PAN chemical solution) containing, forexample, phosphoric acid, acetic acid, and nitric acid. In thispreferred embodiment, the second conductive film is etched with the PANchemical solution containing 70 wt % phosphoric acid, 7 wt % aceticacid, 5 wt % nitric acid, and water. In this stage, the semiconductorchannel layer 7 being the lower layer is protected by the secondto-be-processed insulating film, and thus, is not damaged by thechemical solution.

Then, the second to-be-processed insulating film is selectively etchedto be patterned, using the yet-to-be removed photoresist pattern and thepatterned second conductive film as a mask. The second to-be-processedinsulating film may be dry-etched with a gas containing, for example,fluorine. In this preferred embodiment, the second to-be-processedinsulating film was dry-etched with a gas obtained by adding oxygen (O₂)to sulfur hexafluoride (SF₆). The addition of O₂ can prevent or reducethe damage to the semiconductor channel layer 7 being the lower layercaused by oxidation-reduction reaction during etching. Then, thephotoresist pattern is removed.

Consequently, as shown in FIGS. 7 and 8, in one photolithography process(in the third photolithography process of this example), the protectiveinsulating layers 9, 10, and 11 on the semiconductor channel layer 7 andthe protective insulating layer 8 are formed by patterning secondto-be-processed insulating film and an electrode body 28, the sourcewire 18 a, and the source terminal 19 a are formed by patterning thesecond conductive film. Through the above-mentioned patterning of thesecond to-be-processed insulating film, the first contact hole 14 isformed over the source region of the semiconductor channel layer 7 andthe second contact hole 15 is formed over the drain region of thesemiconductor channel layer 7.

The electrode body 28 mentioned above is the structure including thefirst source electrode 12 located on the protective insulating layer 10and the end portion of the protective insulating layer 9 on theprotective insulating layer 10 side and including the first drainelectrode 13 located on the protective insulating layer 11 and the endportion of the protective insulating layer 9 on the protectiveinsulating layer 11 side. The first source electrode 12 and the firstdrain electrode 13 are continuous with each other on the protectiveinsulating layer 9. As shown in FIG. 8, the portions of the electrodebody 28 corresponding to the first source electrode 12 and the firstdrain electrode 13 are not located directly on the semiconductor channellayer 7 and are located over the semiconductor channel layer 7 with theprotective insulating layers 9, 10, and 11 located therebetween.

Then, a third conductive film being the material of the second sourceelectrode 16 and the second drain electrode 17 is deposited. The secondsource electrode 16 and the second drain electrode 17 electricallyconnect the semiconductor channel layer 7, the first source electrode12, and the first drain electrode 13. In this preferred embodiment, thethird conductive film is an ITO film being a light-transmissiveoxide-based conductive film (in which the mixing ratio (weightpercentage) of In₂O₃ and SnO₂ is 90:10). In general, ITO films have thecrystalline (polycrystalline) structure with stability at roomtemperature. In this example, an ITO film having a thickness of 100 nmand being in the amorphous state is deposited by sputtering, using amixture gas of argon (Ar) gas and a gas containing hydrogen (H) such ashydrogen (H₂) gas or water vapor (H₂O).

Next, in a fourth photolithography process, a photoresist pattern isformed and the third conductive film (the amorphous ITO film) isselectively etched to be patterned, using the photoresist pattern as amask. The third conductive film may be wet-etched with anoxalic-acid-based chemical solution containing, for example, 5 wt %oxalic acid and water.

Then, the electrode body 28 is selectively etched, using the yet-to-beremoved photoresist pattern and the patterned third conductive film as amask. Consequently, the portion of the electrode body 28 located on theprotective insulating layer 9 and continuous with the first sourceelectrode 12 and the first drain electrode 13 is removed. Thus, thesource electrodes (the first and second source electrodes 12 and 16) areelectrically isolated from the drain electrodes (the first and seconddrain electrodes 13 and 17). The electrode body 28 may be wet-etchedwith, for example, a PAN chemical solution. Then the photoresist patternis removed.

Consequently, as shown in FIGS. 9 and 10, in another photolithographyprocess (in the fourth photolithography process of this example), thesecond source electrode 16, the second drain electrode 17, the pixelelectrode 20 being the portion of the second drain electrode 17corresponding to the pixel display region, the source wire 18 b, and thesource terminal 19 b are formed by patterning the third conductive filmand the first source electrode 12 and the first drain electrode 13 areformed by patterning the electrode body 28.

The second source electrode 16 and the second drain electrode 17 thatare made of an oxide conductor such as ITO are directly connected withthe semiconductor channel layer 7 being the lower layer through therespective contact holes being the first contact hole 14 and the secondcontact hole 15. The oxidation-reduction reaction in the source regionand the drain region of the semiconductor channel layer 7 is preventedor further reduced in this configuration than in the configuration inwhich the first source electrode 12 and the first drain electrode 13made of a conductor other than oxide conductors are directly connectedwith the semiconductor channel layer 7. This can prevent or reduce thedegradation of the characteristics of the semiconductor channel layer 7.

As described above, the second source electrode 16, the second drainelectrode 17, and the pixel electrode 20 are formed, and then, a thirdto-be-processed film being the material of the third insulating film 21is deposited on the entire upper surface of the insulating substrate 1.In this preferred embodiment, a silicon oxide (SiO) film having athickness of 200 nm and a silicon nitride (SiN) film having a thicknessof 100 nm were deposited as the third to-be-processed film by CVD in thestated order.

Then, in a fifth photolithography process, a photoresist pattern isformed and the third to-be-processed insulating film and the firstinsulating film 6 are selectively etched to be pattered, using thephotoresist pattern as a mask. The third to-be-processed insulating filmand the first insulating film 6 may be dry-etched with a gas containing,for example, fluorine. In this preferred embodiment, the thirdto-be-processed insulating film and the first insulating film 6 weredry-etched with a gas obtained by adding oxygen (O₂) to sulfurhexafluoride (SF₆). Then, the photoresist pattern is removed.Consequently, as shown in FIGS. 11 and 12, the third insulating film 21having the gate contact hole 22 in the gate terminal portion, the sourcecontact hole 23 in the source terminal portion, and the common electrodecontract hole 24 in the common electrode connection portion are formedby patterning the third to-be-processed insulating film.

Then, a fourth conductive film is deposited. The fourth conductive filmis the material of the gate terminal pad 25 electrically connected withthe gate terminal 4 through the gate contact hole 22, the sourceterminal pad 26 electrically connected with the source terminals 19 aand 19 b through the source contact hole 23, and the counter electrode27 electrically connected with the common wire 5 through the commonelectrode contact hole 24. In this preferred embodiment, alight-transmissive conductive film is deposited as the fourth conductivefilm. The light-transmissive conductive film is an ITO film (in whichthe mixing ratio (weight percentage) of In₂O₃ and SnO₂ is 90:10). Ingeneral, ITO films have the crystalline (polycrystalline) structure withstability at room temperature. In this example, an ITO film having athickness of 100 nm and being in the amorphous state is deposited bysputtering using a mixture gas of argon (Ar) gas and a gas containinghydrogen (H) such as hydrogen (H₂) gas or water vapor (H₂O).

Next, in a sixth photolithography process, a photoresist pattern isformed and the fourth conductive film (the amorphous ITO film) isselectively etched to be patterned, using the photoresist pattern as amask. The fourth conductive film may be wet-etched with anoxalic-acid-based chemical solution containing, for example, 5 wt %oxalic acid and water. Then, the photoresist pattern is removed.

Consequently, as shown in FIGS. 1 and 2, the gate terminal pad 25electrically connected with the gate terminal 4 through the gate contacthole 22, the source terminal pad 26 electrically connected with thesource terminals 19 a and 19 b through the source contact hole 23, andthe counter electrode 27 electrically connected with the common wire 5through the common electrode contact hole 24 are formed by patterningthe fourth conductive film. The slit opening 27 a is provided in thecounter electrode 27 through the above-mentioned patterning.Consequently, the TFT substrate shown in FIGS. 1 and 2 for use in theFFS LCD is completed.

During the assembly of the liquid crystal display panel, an alignmentfilm and a spacer are formed on the surface of the completed TFTsubstrate. The alignment film is the film for aligning liquid crystalsand is made of, for example, polyimide. The counter substrate includinga color filter and an alignment film that have been individuallyproduced is faced with and bonded to the TFT substrate according to thispreferred embodiment. At this time, the TFT substrate and the countersubstrate have a clearance formed by the spacer located therebetween.The liquid crystal layer is formed and sealed in the clearance.Consequently, the liquid crystal panel in the FFS mode is formed. Thus,the liquid crystal display including such liquid crystal display panelcan drive liquid crystals through, from among the electric fieldsgenerated between the counter electrode 27 and the pixel electrode 20,the transverse electric field generated opposite to the pixel electrode20 relative to the counter electrode 27.

Finally, polarizing plates, phase difference plates, and a backlightunit are mainly disposed on the outer side of the liquid crystal displaypanel, so that the liquid crystal display is completed.

<Summary of Preferred Embodiment>

The TFT substrate and the liquid crystal display according to thispreferred embodiment described above has the following configuration.The first source electrode 12 and the first drain electrode 13 made of aconductor (such as a metal layer or an alloy layer) other than oxideconductors are located over the semiconductor channel layer 7 made of anoxide semiconductor with the protective insulating layers 9, 10, and 11located therebetween. Such configuration can prevent or reduce thedamage to the semiconductor channel layer 7 during the processing forforming the source electrode and the drain electrode. Thus, thisconfiguration can enhance the characteristics and the reliability of theTFTs.

In this preferred embodiment, the second source electrode 16 and thesecond drain electrode 17 made of an oxide conductor providesconnections between the semiconductor channel layer 7 and the firstsource electrode 12 and between the semiconductor channel layer 7 andthe first drain electrode 13. The first source electrode 12 and thefirst drain electrode 13 are made of a conductor other than oxideconductors. Unlike the conventional technique, this configuration canprevent or reduce the oxidation-reduction reaction occurring between thesemiconductor channel layer 7 and the first source electrode 12 andbetween the semiconductor channel layer 7 and the first drain electrode13. Consequently, the contact properties in the relevant interfaces areimproved, resulting in the improvement in the on-state current value andthe value of mobility. This enhances the characteristics and thereliability of the TFTs.

If the semiconductor channel layer 7 extends off the periphery (the edgeportion) of the gate electrode 2 in plan view, the breakdown voltage inthe step portion formed in the periphery of the gate electrode 2 due tothe thickness of the gate electrode 2 could be deteriorated or steppedcuts (wire breakage or ruptures associated with the step portion) couldappear in the source electrode and the drain electrode formed above thestep portion.

In this preferred embodiment, meanwhile, the semiconductor channel layer7 is located inside the periphery of the gate electrode 2 in plan view.Thus, the first insulating film 6 and the protective insulating layers10 and 11 are formed above the step portion of the gate electrode 2,thereby improving the breakdown voltage between the gate electrode 2 andthe source electrode and between the gate electrode 2 and the drainelectrode. The protective insulating layers 9, 10, and 11 are formed tocover the semiconductor channel layer 7, thereby preventing or reducingthe electrical short circuit defects between the gate electrode 2 andthe first source electrode 12. The coverage properties (the coveringability) of the source electrode and the drain electrode are enhanced,which prevents or reduces the stepped cut defects of the sourceelectrode and the drain electrode associated with the step portion.

Under the method for manufacturing TFT substrate according to thispreferred embodiment, the protective insulating layers 9, 10, and 11 andthe electrode body 28 that is to be formed into the first sourceelectrode 12 and the first drain electrode 13 are formed in onephotolithography process. Then, the second source electrode 16, thesecond drain electrode 17, and the pixel electrode 20 as well as thefirst source electrode 12 and the first drain electrode 13 are formed inanother photolithography process. Such manufacturing method eliminatesthe need for performing the photolithography dedicated to the formationof the protective insulating layers 9, 10, and 11 that protect thesemiconductor channel layer 7. This controls the decrease in productioncapacity and the increase in the manufacturing cost associated with theincrease in the number of masks for the photolithography.

As described above, in this preferred embodiment, the first sourceelectrode 12 and the first drain electrode 13 made of a conductor otherthan oxide conductors are located over the semiconductor channel layer 7made of an oxide semiconductor with the protective insulating layers 9,10, and 11 located therebetween. This can prevent or reduce the damageto the semiconductor channel layer 7 resulting from the film depositionand etching during the formation of the first source electrode 12 andthe first drain electrode 13, thus enhancing the characteristics and thereliability of the TFTs. The use of the oxide semiconductor film havinga higher mobility as the semiconductor channel layer 7 of the TFT allowsfor the high-yield manufacturing of a TFT substrate having a higheroperating speed and a liquid crystal display including the TFTsubstrate. Thus, high-performance TFT substrates and liquid crystaldisplays can be manufactured with high productivity.

<Modification>

With reference to FIGS. 13 and 14, the following describes theconfiguration of a TFT substrate according to a modification of thispreferred embodiment. FIG. 13 is a view of the planar configuration ofthe pixel portion as well as the gate terminal portion and the sourceterminal portion outside the pixel portion (the pixel display region) ofthe TFT substrate in the FFS mode according to this modification. FIG.14 is a view of the cross-sectional configuration of these portions.FIG. 14 shows cross sections taken along the line X1-X2, the line Y1-Y2,and the line Z1-Z2 in FIG. 13. The cross section taken along the lineX1-X2 corresponds to the region (the pixel portion) in which pixels arelocated. The cross section taken along the line Y1-Y2 corresponds to theregion (the gate terminal portion) in which the gate terminal 4 and thegate terminal pad 25 for supplying a gate signal to the gate wire 3 (notshown in FIG. 14) are located. The cross section take along the lineZ1-Z2 corresponds to the region (the source terminal portion) in whichthe source terminals 19 a and 19 b and the source terminal pad 26 forapplying a display signal to the source wires 18 a and 18 b (not shownin FIG. 14) are located.

In this modification, at least one of the common wire 5 and the pixelelectrode 20 in the first preferred embodiment has a pattern that ispartially changed in such a manner that at least a part of the pixelelectrode 20 overlaps at least a part of the common wire in plan view.The formation of such overlapping region adds the auxiliary capacitanceto the liquid crystal capacitance of the pixel electrode 20, therebyimproving the retention properties of retaining electric charges of animage signal written upon the pixel electrode 20. Consequently, thedisplay defects such as uneven brightness can be prevented or reduced,thus providing the liquid crystal display panel (liquid crystal display)in the FFS mode having higher display quality.

In the first preferred embodiment and the modification mentioned above,the application of the present invention to the liquid crystal displaypanel (liquid crystal display) in the FFS mode has been described as anexample. The structure associated with the TFT portion and the pixelelectrode 20 connected with the TFT portion is applicable not only tothe FFS LCDs but also to, for example, the transmissive LCDs in thetwisted nematic (TN) mode that include pixel electrodes made of an oxideconductor.

The transmissive LCDs in the TN mode having the present inventionapplied thereto has the same configuration as that of the firstpreferred embodiment including the basic configuration of the TFTportion and the structure for connecting the TFT portion with the pixelelectrode 20 except that the configuration of the common wire 5 and thecounter electrode 27 located over the TFT substrate are omitted. Inplace of the counter electrode 27 located over the TFT substrate, acounter electrode made of an oxide conductor such as an ITO film islocated across the entire pixel display region on the counter substrateincluding a color filter and an alignment film. This configuration ispartially changed in such a manner that liquid crystals are driven bythe electric field generated between the pixel electrode 20 located onthe TFT substrate and the counter electrode located on the countersubstrate.

Such configuration changed as described above also produces the basiceffects as in the first preferred embodiment. Thus, this configurationcan prevent or reduce the damage to the semiconductor channel layer 7resulting from the film deposition and etching during the formation ofthe first source electrode 12 and the first drain electrode 13, therebyproducing the effect of enhancing the characteristics and thereliability of the TFTs. As another effect of this configuration, theuse of the oxide semiconductor film having a higher mobility as thesemiconductor channel layer 7 of the TFT allows for the high-yieldmanufacturing of a TFT substrate having a higher operating speed and aliquid crystal display including the TFT substrate. Thus, as in thefirst preferred embodiment, high-performance TFT substrates and liquidcrystal displays can be manufactured with high productivity.

In the present invention, the above preferred embodiment can beappropriately varied or omitted within the scope of the invention.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A thin film transistor substrate comprising: aninsulating substrate; a gate electrode located on said insulatingsubstrate; a first insulating film that covers said gate electrode; asemiconductor film located over said gate electrode with said firstinsulating film located therebetween, said semiconductor film beingformed of an oxide semiconductor; a second insulating film that coverssaid semiconductor film; a first source electrode located on one end ofsaid second insulating film, said first source electrode being formed ofa conductor other than oxide conductors; a first drain electrode locatedon another end of said second insulating film, said first drainelectrode being formed of a conductor other than oxide conductors; asecond source electrode located on said first source electrode andconnected with said semiconductor film through a first contact holeprovided in said first source electrode and said second insulating film,said second source electrode being formed of an oxide conductor; asecond drain electrode located on said first drain electrode andconnected with said semiconductor film through a second contact holeprovided in said first drain electrode and said second insulating film,said second drain electrode being formed of an oxide conductor; a pixelelectrode formed by extending a part of said second drain electrode; anda third insulating film that covers said first insulating film, saidsecond source electrode, said second drain electrode, and said pixelelectrode.
 2. The thin film transistor substrate according to claim 1,further comprising: a common wire located on said insulating substrateand covered with said first insulating film; and a counter electrodelocated over said pixel electrode with said third insulating filmtherebetween and connected with said common wire through a third contacthole provided in said first insulating film and said third insulatingfilm, said counter electrode being capable of generating a transverseelectric field between said pixel electrode and said counter electrode.3. The thin film transistor substrate according to claim 1, wherein saidsemiconductor film is located inside a periphery of said gate electrodein plan view.
 4. The thin film transistor substrate according to claim2, wherein said pixel electrode is located on said first insulatingfilm, and at least a part of said pixel electrode overlaps at least apart of said common wire in plan view.
 5. A liquid crystal displaycomprising: a thin film transistor substrate; and a counter substrateopposed to said thin film transistor substrate, wherein said thin filmtransistor substrate includes: an insulating substrate; a gate electrodelocated on said insulating substrate; a first insulating film thatcovers said gate electrode; a semiconductor film located over said gateelectrode with said first insulating film located therebetween, saidsemiconductor film being formed of an oxide semiconductor; a secondinsulating film that covers said semiconductor film; a first sourceelectrode located on one end of said second insulating film, said firstsource electrode being formed of a conductor other than oxideconductors; a first drain electrode located on another end of saidsecond insulating film, said first drain electrode being formed of aconductor other than oxide conductors; a second source electrode locatedon said first source electrode and connected with said semiconductorfilm through a first contact hole provided in said first sourceelectrode and said second insulating film, said second source electrodebeing formed of an oxide conductor; a second drain electrode located onsaid first drain electrode and connected with said semiconductor filmthrough a second contact hole provided in said first drain electrode andsaid second insulating film, said second drain electrode being formed ofan oxide conductor; a pixel electrode formed by extending a part of saidsecond drain electrode; and a third insulating film that covers saidfirst insulating film, said second source electrode, said second drainelectrode, and said pixel electrode, wherein said liquid crystal displaydrives liquid crystals through an electric field generated between saidpixel electrode and a counter electrode located on said thin filmtransistor substrate or said counter substrate.
 6. The liquid crystaldisplay according to claim 5, further comprising: a common wire locatedon said insulating substrate and covered with said first insulatingfilm; and a counter electrode located over said pixel electrode withsaid third insulating film therebetween and connected with said commonwire through a third contact hole provided in said first insulating filmand said third insulating film, said counter electrode being capable ofgenerating a transverse electric field between said pixel electrode andsaid counter electrode.
 7. The liquid crystal display according to claim5, wherein said semiconductor film is located inside a periphery of saidgate electrode in plan view.
 8. The liquid crystal display according toclaim 6, wherein said pixel electrode is located on said firstinsulating film, and at least a part of said pixel electrode overlaps atleast a part of said common wire in plan view.